1. Technical Field
The present disclosure relates to systems and methods for analyzing flow waveforms in the peripheral vasculature, e.g., for assessing changes in blood volume.
2. Background Art
The present disclosure expands on and extends the teachings of U.S. Patent Publication No. 2007/0032732 to Shelley et al., entitled “Method of Assessing Blood Volume using Photoelectric Plethysmography” (referred to herein as the “Shelley Publication”). Accordingly, the foregoing patent publication is incorporated herein in its entirety.
Traditionally, invasive monitoring has been required to detect decreases in intravascular volume. In recent years, however, intraoperative monitoring has been moving towards minimally-invasive or non-invasive techniques. This shift has been attributed to various considerations, including procedure time, cost, and known risks which for traditionally invasive techniques may include carotid artery puncture, arrhythmia, pneumothorax, and infection. Indeed, there is growing evidence that invasive monitors of volume status, such as the pulmonary artery catheter (PAC), may be a source of unacceptably frequent complications. Dalen J & Bone R, Is it time to pull the pulmonary artery catheter?, JAMA 1996; 276:916-14; Connors A, Speroff T & Dawson N, The effectiveness of right heart catheterization in the initial care of critically ill patients, JAMA 1996; 276:889-97. Thus, insertion of such monitors for the sole purpose of monitoring volume status often is withheld so as to avoid iatrogenic complications as well as to reduce costs and delays. In addition the accuracy and clinical usefulness of these monitors have been questioned. See e.g., Bouchard, M. J., et al., Poor correlation between hemodynamic and echocardiographic indexes of left ventricular performance in the operating room and intensive care unit. Crit. Care Med, 2004. 32(3): p. 644-8. The potential loss of this and other important monitors from routine perioperative care necessitates the search for another means of monitoring a patient's blood volume status. Hence, there is a need for minimally-invasive or non-invasive alternative systems and methods for assessing a patient's volume status, particularly in emergency, preoperative, and intensive care settings. Ideally, such systems and methods should be able to detect decreases in blood before major complications develop (e.g., before decreased blood pressure).
The Plethysmographic Waveform:
To meet these needs, investigators have been pursuing methods for assessing blood volume based on cardiovascular waveforms which are detectable in the peripheral vasculature. One such waveform is the plethysmographic (PG) waveform as may be obtained, e.g., via a pulse oximeter. In the process of determining oxygen saturation, a pulse oximeter inherently functions as a photoplethysmograph, measuring minute changes in the blood volume of a vascular bed (e.g., finger, ear or forehead). Thus, while the predominant application of a pulse oximeter has been calculating oxygen saturation of Hb, it is noted that the raw PG waveform is rich in information relevant to the physiology of the patient. Indeed, the PG waveform contains a complex mixture of the influences of arterial, venous, autonomic and respiratory systems on the peripheral circulation. It is important to understand, however, that the typical pulse oximeter waveform presented to the clinician is a highly filtered and processed specter of the original PG signal. Indeed, it is normal practice for equipment manufacturers to use both auto-centering and auto-gain routines on the displayed waveforms so as to minimize variations in the displayed signal. While such signal processing may be beneficial to the determination of oxygen saturation, it often comes at the expense of valuable physiological data. Thus, due to a general lack of access to the raw PG waveform and the overriding clinical importance of monitoring oxygen saturation, various other potential uses for the PG waveform have been largely neglected.
It is disclosed in the literature that a PG can be used to non-invasively measure minute changes in light absorption of living tissue. See, e.g., Hertzman, A B, “The Blood Supply of Various Skin Areas as Estimated By the Photoelectric Plethysmograph,” Am. J. Physiol. 124: 328-340 (1938). Rhythmic fluctuations in this signal are normally attributed to the cardiac pulse bringing more blood into the region being analyzed (e.g., finger, ear or forehead). This fluctuation of the PG signal is commonly referred to as the pulsatile or AC (arterial) component. The amplitude of the AC component can be modulated by a variety of factors, including cardiac stroke volume and vascular tone. In addition to the pulsatile component of the PG signal, there is a nonpulsatile (or weakly pulsatile) component of the PG signal commonly referred to as the DC component. The DC component is most commonly attributed to changes in light absorption by nonpulsatile tissue, such as fat, bone, muscle and venous blood. Thus, the DC component has been correlated to changes in venous blood volume (see, e.g., paragraph [0059] of the Shelley publication).
Methods for extracting and analyzing the AC and DC components of the PG signal are provided in the Shelley publication. The ability to independently monitor changes in venous and arterial blood volume has many clinical applications. For example, changes in venous and arterial blood volume may be indicative of Hypovolemia, e.g., due to bleeding, dehydration, etc. Decreased blood volume due to bleeding is, typically, characterized by an initial period of venous loss during which the cardiac output remains unaffected. With continued blood loss, decreased venous return eventually affects cardiac output (corresponding to arterial blood volume).
It is again noted, however, since the main purpose of the pulse oximeter is determination of arterial oxygen saturation, most pulse oximeters filter out the venous (DC) component and normalize the arterial (AC) component to facilitate visualization of the signal. In addition, pulse oximeters are most commonly used on the finger, a region rich in sympathetic innervation that often reflects local (as opposed to systemic) alterations in vascular tone and volume status. See, e.g., Yamakage M, Itoh T, Iwasaki S, Jeong S-W, Namiki A, Can variation of pulse amplitude value measured by a pulse oximeter predict intravascular volume?, Anesthesiology 2004 abstracts; Dorlas J C, Nijiboer J A, Photo-electric plythysmography as a monitoring device in anaesthesia. Application and interpretations, BR J Anaethesia 1999 82(2):178-81; A245, H188, H236.
Peripheral Venous Pressure:
A further largely unexplored source of clinical information is pressure transduction of the standard intravenous line. A vast majority of hospitalized patients have a peripheral venous line. It is placed to allow fluids and medications to be given directly into the circulatory system. Until recently, the venous system's contribution to the circulatory system has been incorrectly identified as being insignificant. Indeed, veins do more than merely conduct blood to the heart; veins play a critical role in cardiovascular homeostasis. Thus, considering the ease of measurement from a peripheral intravenous catheter, further investigation of the utility and limitations of such a minimally invasive and inexpensive monitoring device is warranted.
Folkow, in the 1960s, studied the characteristics of veins and noted the huge disparity which existed in the literature concerning the amount of information on the arterial vs. the venous sides of the circulation. Folkow B, Mellander S., Veins and Venous Tone, Am Heart J. 1964; 68:397-408. Almost 50 years later, we have still not filled the gap. While arterial waveforms have been studied extensively, focus on the peripheral venous component has been scarce.
Controversy still exists concerning the role of peripheral veins and their contribution to the central volume in face of blood loss. Many studies in the late 1990s and early 2000s have shown a consistent correlation between peripheral venous pressure (PVP) and central venous pressure (CVP). See, e.g., Weingarten T N, Sprung J, Munis J R., Peripheral venous pressure as a measure of venous compliance during pheochromocytoma resection, Anesth Analg. 2004; 99:1035-7, table of contents; and Charalambous C, Barker T A, Zipitis C S, Siddique I, Swindell R, Jackson R, et al., Comparison of peripheral and central venous pressures in critically Ill patients, Anaesth Intensive Care. 2003; 31:34-9. While CVP waveforms characteristically show a-, c-, and v-waves, PVP waveforms often appear as a more dampened sinusoidal pattern. Munis et al. reported mean PVP values of 13 mm Hg, CVP values of 10 mm Hg, with a PVP-CVP difference of 3 mm Hg (see Munis J. R., Bhatia et al., Peripheral venous pressure as hemodynamic variable in neurosurgical patients, Anesth Analg 2001; 91(1): 172-9). Amar et al. observed mean PVP values of 9 mm Hg and a mean CVP value of 8 mm Hg in 100 intraoperative patients (see Amar D, Melendez J A, Zhang H, Dobres C, Leung D H, Padilla R E, Correlation of peripheral venous pressure and central venous pressure in surgical patients, J Cardiothorac Vasc Anesth. 2001; 15:40-3). Hadimioglu et al. came to the same conclusions in patients undergoing kidney transplant (see Hadimioglu N, Ertug Z, Yegin A, Sanli S, Gurkan A, Demirbas A, Correlation of peripheral venous pressure and central venous pressure in kidney recipients, Transplant Proc. 2006; 38:440-2). Baty et al studied 29 infants and children post cardiopulmonary bypass. The difference between peripheral venous pressure and central venous pressure in these patients was 11±3 mm Hg. No clinically significant variation in the accuracy of the technique was noted based on the actual CVP value, size of the PIV, its location, or the patient's weight (see Baty L, Russo P, Tobias J D, Measurement of central venous pressure from a peripheral intravenous catheter following cardiopulmonary bypass in infants and children with congenital heart disease, J Intensive Care Med. 2008; 23:136-42).
Other authors have done similar assessments in patients undergoing right hepatectomy. In Choi et al., a central venous catheter was placed through the right internal jugular vein and a peripheral venous catheter was inserted at the antecubital fossa in the right arm. A total of 1,430 simultaneous measurements of CVP and PVP were recorded. Choi concluded the difference between PVP and CVP was within clinically acceptable agreement and the degree of difference tended to remain relatively constant throughout the right hepatectomy in living donors. (See Choi S J, Gwak M S, Ko J S, Kim G S, Kim T H, Ahn H, et al., Can peripheral venous pressure be an alternative to central venous pressure during right hepatectomy in living donors?, Liver Transpl. 2007; 13:1414-21). Hoftman et al. studied the correlation of both variables in patients undergoing liver transplant. The nature of the liver transplant surgery allowed the authors to test the durability of the PVP/CVP correlation during extreme derangements of physiology, including IVC crossclamp, brisk hemorrhage, and reperfusion of the donor graft. One unexpected finding, not previously reported in other studies, was the much weaker PVP/CVP correlation at low filling pressures. It was suggested that at low filling pressures, peripheral veins intermittently collapse, interrupting their continuity with the central circulation and thus leading to PVP/CVP divergence. (See Holtman N, Braunfeld M, Holtman G, Mahajan A., Peripheral venous pressure as a predictor of central venous pressure during orthotopic liver transplantation, J Clin Anesth. 2006; 18:251-5).
According to Munis et al. (2001), PVP may be used as an indirect measure of venous volume since pressure is related to volume/compliance. Alternatively, it was reported that fluctuations of PVP are highly influenced by changes in vascular tone. Thus, measurements of volume status using PVP may be distorted by local changes in vascular tone. Vincent at al. documented that hand vein compliance decreases in responses to the alpha-agonist phenylephrine. Vincent J, et al., Cascular reactivity to phenylephrine and angiotensin II: comparison of direct venous and systemic vascular responses, Clin Pharmocol Ther 1992; 51:68-75.
Moreover, the relationship of peripheral venous pressure and central venous pressure differs among patients. For example, the offset in Munis' study averaged 3.0 mmHg, ranging from 0.5 to 8.9 mmHg over 15 subjects. Similarly, Pederson et al. reported a mean gradient of 2.6 cm H2O and a range of 0.7 to 5.8 cm H2O between the antecubital vein and right atrium. Hence, without a baseline comparison to CVP (which requires invasive insertion of a central venous catheter), it is difficult to determine the accuracy of PVP measurements.
Generally, while there have been attempts to relate PVP to CVP (see, e.g., Eustace B R., A comparison between peripheral and central venous pressure monitoring under clinical conditions, Injury 1970; 2(1):12-18; Choi S J, Gwak M S, Ko J S, Kim G S, Kim T H, Ahn H, et al., Can peripheral venous pressure be an alternative to central venous pressure during right hepatectomy in living donors?, Liver Transpl. 2007; 13:1414-21; Holtman N, Braunfeld M, Holtman G, Mahajan A., Peripheral venous pressure as a predictor of central venous pressure during orthotopic liver transplantation, J Clin Anesth. 2006; 18:251-5; Milhoan K A, Levy D J, Shields N, Rothman A., Upper extremity peripheral venous pressure measurements accurately reflect pulmonary artery pressures in patients with cavopulmonary or Fontan connections, Pediatr Cardiol. 2004; 25:17-9.; Tobias J D, Johnson J O., Measurement of central venous pressure from a peripheral vein in infants and children, Pediatr Emerg Care. 2003; 19:428-30; and Desjardins R, Denault A Y, Belisle S, Carrier M, Babin D, Levesque S, et al., Can peripheral venous pressure be interchangeable with central venous pressure in patients undergoing cardiac surgery?, Intensive Care Med. 2004; 30:627-32), very little effort has been made to characterize the PVP waveform as an independent entity.
In the past, a number of investigators have advanced the concept that a small change in venous capacity, induced by venous constriction or relaxation, should markedly alter the cardiac output. See, e.g., Bartelstone H J., Role of the veins in venous return, Circ Res. 1960; 8:1059-76. In a delicately designed experiment involving dogs, Bartelstone was able to divide the venous system into two major components: (1) the central venous conduit, holding approximately 18% of the total blood volume and including the inferior Vena Cava (IVC) and the large vein continuations thereof, and (2) the reactive venous reservoir, containing approximately 45% of the total blood volume and including the veins between the capillaries and the central venous conduit. Bartelstone was also able to demonstrate that there exists an intravenous gradient which facilitates the movement from the reactive venous reservoir to the central venous conduit. Bartelstone further displayed that sympathetic stimulation had no significant impact on the central venous conduit, despite a dynamic impact on the reactive venous reservoir.
Venous Compliance:
Rothe in the 1990s effectively tackled the issue of compliance in the venous compartment. Thus, Rothe illustrated the concept of Mean Circulatory Filling Pressure (PMCF) described first by Guyton. He defined PMCF as mean vascular pressure that exists after circulatory arrest leading to redistribution of blood, so that all pressures are the same throughout the system. PMCF is thus related to the fullness of the circulatory system. This pressure has been measured and found to be close to 7 mm of Hg. This is clearly less than capillary pressure, but it is greater than the venous pressure at the atrio-caval junction under normal conditions. See Rothe C F, Mean circulatory filling pressure: its meaning and measurement, J Appl Physiol. 1993; 74:499-509.
As is evident from FIG. 1P, there is a huge contrast between venous and arterial compliance. The enormous compliance of veins allows for huge shifts of circulating volume in and out of the venous compartment. Peripheral venous constriction, as evidenced by the dashed line, tends to increase venous pressure and shift blood out of the venous compartment. Mohrman D, Cardiovascular Physiology. 6th ed. New York: McGraw-Hill Medical; 2006.
Two primary factors are known to affect peripheral venous tone: (1) blood volume within the veins: because the veins are so much more compliant, changes in circulating blood volume produce larger changes in the volume of blood in the veins than in any other vascular segment. Tyberg J V, How changes in venous capacitance modulate cardiac output, Pflugers Arch. 2002; 445:10-7; and (2) sympathetic venous activity. In addition, an increase in any force compressing veins from the outside has the same effect on the pressure inside veins as an increase in venous tone. Thus, such things as muscle exercise and wearing elastic stockings tend to elevate peripheral venous pressure.
The relationship between central venous pressure and venous return is known as the Venous Return Curve (see FIG. 2P). When venous tone changes, so does the central venous pressure. For example, whenever peripheral venous pressure is elevated by increases in blood volume or by sympathetic stimulation, the venous function curve shifts upward and to the right.
Mohrman D 2006. This is believed to be caused by a decrease in venous capacitance which raises the mean circulatory pressure, which in turn tends to increase all intravascular pressures, and thus increases the preload of the heart. Id.
In the year 1955, Guyton, a man known for his valuable contributions to the field of physiology, explained the relationship between venous compliance and cardiac output. He used Starling's law for the determination of cardiac output which he defined as the relationship between the cardiac output and right atrial pressure and called the “cardiac response curve”. Guyton A C, Determination of cardiac output by equating venous return curves with cardiac response curves, Physiol Rev. 1955; 35:123-9
FIG. 3P demonstrates that peripheral venous constriction increases cardiac output by raising central venous pressure and moving the heart's function upward along a fixed cardiac function curve. FIG. 3P also depicts the response of the vasculature to hemorrhage into progressive steps (i.e., A to B to C to D) which does not happen discretely in reality. The actual course of a patient's net response to hemorrhage would appear to follow nearly a straight line from point A to point D.
The behavior of peripheral veins of the forearm, in response to hemorrhage or sympathetic activity, is conflicting. While Zoller was able to demonstrate that the forearm veins show intense venoconstriction in the absence of changes in other hemodynamic parameters, other studies have proved that those limb veins have very little role to play in contributing to the central blood volume. Zoller R P, Mark A L, Abboud F M, Schmid P G, Heistad D D, The role of low pressure baroreceptors in reflex vasoconstrictor responses in man, J Clin Invest. 1972; 51:2967-72.
Ventilation-Induced Variation
It has been known for quite some time that ventilation, and especially positive pressure ventilation, can have a significant impact on the cardiovascular system. Cournand A, Modey H, Werko L & Richards D, Physiological studies of the effect of intermittent positive pressure breathing on cardiac output in man, Am J Physio 1948; 152:162-73; Morgan B, Crawford W & Guntheroth W, The homodynamic effects of changes in blood volume during intermittent positive-pressure ventilation, Anesthesiology 1969; 30:297-305. The first formal studies of the effect of ventilator induced changes on arterial pressure were done in the early 1980's. Coyle J, Teplick R, Long M & Davison J. Respiratory variations in systemic arterial pressure as an indicator of volume status, Anesthesiology 1983; 59:A53; Jardin F, Fareot J, Gueret P et al., Cyclic changes in arterial pulse during respiratory support, Circulation 1983; 68:266-74. This recognition was soon followed by the intensive investigations of Azriel Perel who coined the term “systolic pressure variation” to describe this phenomenon. Along with various co-investigators, his research has encompassed over twenty articles and abstracts on the topic. From this significant body of work, based on both animal and human data, a number of conclusions have been drawn.
It has been shown that the responses of peripheral waveforms to respiration can be used as an indicator of hypovolemia. More specifically, arterial pressure waveforms in the periphery (e.g., radial artery) demonstrate increased systolic pressure variations in the context of hypovolemia (as a result of ventilation affecting venous return to the heart and hence affecting left ventricular stroke volume). The degree of systolic pressure variation is a sensitive indicator of hypovolemia. Perel A, Pizov R & Cotev S, Systolic blood variation is a sensitive indicator of hypovolemia in ventilated dogs subjected to graded hemorrhage, Anesthesiology 1987; 67:498-502. This variation is significantly better than heart rate, central venous pressure and mean systemic blood pressure in predicting the degree of hemorrhage which has occurred. Perel A, Pizov R & Cotev S, Systolic blood pressure variation is a sensitive indicator of hypovolemia in ventilated dogs subjected to graded hemorrhage, Anesthesiology 1987; 67:498-502; Pizov R, Ya'ari Y & Perel A, Systolic pressure variation is greater during hemorrhage than during sodium nitroprusside-induced hypotension in ventilated dogs, Anesthesia & Analgesia 1988; 67:170-4. Chest wall compliance and tidal volume can influence systolic pressure variation. Szold A, Pizov R, Segal E & Perel A, The effect of tidal volume and intravascular volume state on systolic pressure variation in ventilated dogs, Intensive Care Medicine 1989; 15:368-71. Changes in systolic pressure variation correspond closely to changes in cardiac output. Ornstein E, Eidelman L, Drenger B et al., Systolic pressure variation predicts the response to acute blood loss, Journal of Clinical Anesthesia 1998; 10:137-40; Pizov R, Segal E, Kaplan L et al., The use of systolic pressure variation in hemodynamic monitoring during deliberate hypotension in spine surgery, Journal of Clinical Anesthesia 1990; 2:96-100.
Systolic pressure variation can be divided into two distinct components; Δup, which reflects an inspiratory augmentation of the cardiac output, and Δdown, which reflects a reduction in cardiac output due to a decrease in venous return. Perel A, Cardiovascular assessment by pressure waveform analysis, ASA Annual Refresher Course Lecture 1991:264. The unique value in systolic pressure variation lies in its ability to reflect the volume responsiveness of the left ventricle. Perel A, Cardiovascular assessment by pressure waveform analysis, ASA Annual Refresher Course Lecture 1991:264. In recent years, with the increased availability of the pulse oximeter waveform, similar observations have been made with this monitoring system. Partridge B L, Use of pulse oximetry as a noninvasive indicator of intravascular volume status, Journal of Clinical Monitoring 1987; 3:263-8; Lherm T, Chevalier T, Troche G et al., Correlation between plethysmography curve variation (dpleth) and pulmonary capillary wedge pressure (pcup) in mechanically ventilated patients, British Journal of Anesthesia 1995; Suppl. 1:41; Shamir M, Eidelman L A et al., Pulse oximetry plethysmographic waveform during changes in blood volume, British Journal Of Anaesthesia 82(2): 178-81 (1999).
To date, however, there has been remarkably little work done to document or quantify the phenomenon of systolic pressure variation. Limitations of the aforementioned include, inter alia, reliance on positive pressure and mechanical ventilation; and the requirement of ventilator maneuvers, such as periods of apnea.
As for detecting systolic pressure variation, it is noted that changes in intrathoracic pressure during ventilation causes variations in the PG signal. Fluctuations in the PG signal due to respiration/ventilation can be detected. See, e.g., Johansson A & Oberg P A, “Estimation of respiratory volumes from the photoplethysmographic sit. Parti: Experimental results,” Medical and Biological Engineering and Computing 37(1): 42-7 (1999). Respiratory-induced fluctuations have been used in the past in an attempt to estimate the degree of relative blood volume of patients undergoing surgery. See, e.g., Partridge B L, “Use of pulse oximetry as a noninvasive indicator of intravascular volume status,” Journal of Clinical Monitoring 3(4): 263-8 (1987); and Shamir M, Eidelman L A et al., “Pulse oximetry plethysmographic waveform during changes in blood volume,” British Journal of Anaesthesia 82(2): 178-81 (1999).
In the Shelley publication, it was first noted that respiration/ventilation modulates both the AC and DC components of the PG signal. Thus, the Shelley publication discloses, inter alia, methods for monitoring blood volume by separating the impact of ventilation on the arterial and venous systems. The ability to mathematically separate the impact of ventilation on the arterial (pulsatile component) and venous (nonpulsatile component) systems allows one to independently assess changes in blood volume in two different regions of the vasculature (arterial and venous). Venous blood volume corresponds to end-diastolic volume (EDV) (also referred to as preload). EDV directly impacts the amount of blood available to the heart before each contraction. Arterial blood volume corresponds to cardiac stroke volume. Cardiac stroke volume may be calculated by subtracting end-systolic volume (ESV) from EDV. Cardiac output is determined as cardiac stroke volume multiplied by heart rate.
As noted in the Shelley publication, the degree of respiratory-induced fluctuation of the DC component of the PG signal corresponds to venous blood volume. Thus, by monitoring respiratory-induced fluctuations of the DC component one can detect and counter blood loss (e.g., hypovolemia) prior to cardiac output being affected. Alternatively, one can detect and counter over fluidization (e.g., hypervolemia). Similarly, as noted in the Shelley publication, the degree of respiratory-induced fluctuation of the AC component of the PG signal corresponds to arterial volume. Thus, by monitoring respiratory-induced fluctuations of the AC component, one can detect the severity of blood loss (i.e., whether blood loss is severe enough to compromise cardiac function).
An alternate method suggested by the Shelley publication for assessing changes in blood volume involves harmonic analysis, e.g., Fourier analysis, of the PG waveform. Harmonic analysis allows for the extraction of underlying signals that contribute to a complex waveform. Similar methods have been used, e.g., to improve the accuracy of oxygen saturation measurements and to monitor tissue perfusion. See, e.g., Rusch T L, Sankar R & Scharf J E, “Signal processing methods for pulse oximetry,” Computers in Biology and Medicine 1996; 26:143-59; and Stack B Jr., Futran N D, Shohet M N & Scharf J E, “Spectral analysis of photoplethysmograms from radial forearm free flaps,” Laryngoscope 1998; 108:1329-33.
Harmonic analysis of the PG waveform, as disclosed in the Shelley publication, principally involves a short-time Fourier transform of the PG signal. In particular, the PG waveform may be converted to a numeric series of data points via analog to digital conversion, wherein the PG signal is sampled at a predetermined frequency, e.g., 50 Hz, over a given time period, e.g., 60-90 seconds. A Fourier transform may then be performed on the data set in the digital buffer (note that the sampled PG signal may also be multiplied by a windowing function, e.g., a Hamming window, to counter spectral leakage). The resultant data may further be expanded in logarithmic fashion, e.g., to account for the overwhelming signal strength of the cardiac frequencies relative to the ventilation frequencies. While the Shelley publication discloses joint time-frequency analysis, i.e., a spectrogram, as a preferred technique for viewing and analyzing spectral density estimation of the PG signal, the spectrum of the PG signal over a set period of time may be easily extrapolated therefrom.
According to the Shelley publication, harmonic analysis, such as described above, may be used to independently monitor changes in arterial and venous blood volume. For instance, an initial increase in signal strength for the respiratory signal is observed to be largely due to increased respiratory-induced fluctuation of the DC component of the PG signal indicative of venous loss (note that although an initial increase in the respiratory signal is reflective of venous loss, subsequent decreased cardiac output (e.g., resulting when decreased venous return affects the arterial system) may also contribute to changes in the respiratory signal). Similarly, changes in blood volume severe enough to affect the arterial system (cardiac output) were correlated to increased side-band modulation around the primary band of the cardiac signal. Thus, by monitoring variations in the respiratory signal one is able to detect changes in venous blood volume. Similarly, by monitoring side-band modulation of the cardiac signal one is able to detect changes in arterial blood volume.
Analysis of venous waveforms has indicated that, like arterial waveforms, they too exhibit respiratory variations and change in response to physiologic challenges. Brecher et al. examined the relationship of respiration on the intrathoracic (the central venous conduit) and extrathoracic veins (the reactive venous reservoir). Brecher et al. conducted experiments using both spontaneously breathing and mechanically ventilated dogs. Pressure recordings were obtained from the jugular vein, femoral artery, intrapleural space and right atrium. Brecher concluded the following for spontaneous breathing under normal volume status: (1) thoracic aspiration during inspiration causes increase in blood flow to the right atrium significantly due to the emptying of the extrathoracic veins into the central veins; (2) flow does not increase further once the collapsed state of extrathoracic veins has been reached; and (3) if inspiration is long and deep enough, flow may even drop slightly below its inspiratory maximum due to the exhaustion of the extrathoracic reservoir and the progressively increasing resistance offered by the partially collapsed extrathoracic veins. Brecher then studied the same relationship under conditions of hyper and hypovolemia and concluded that identical degrees of thoracic aspiration increase venous return only moderately in the hypovolemic state as compared to euvolemic state. Brecher further noted that the greater the hypovolemia, the shorter the duration and amount of the aspiratory flow augmentation and the earlier the onset of the collapsed stage. (See Brecher GA, Mixter G, Jr., Effect of respiratory movements on superior cava flow under normal and abnormal conditions, Am J. Physiol. 1953; 172:457-61).
Respiratory variations in the central venous waveform have been described before. The respiratory induced variation in central vein pressure also causes variations in arterial blood pressure (ABP), as described above, and in peripheral venous pressure (PVP). Valves in the venous system in the forearm may hinder hydrostatic continuity, implying that one single vein might not represent the entire venous system in the forearm. Whether the respiratory variation in PVP is a forward transmission of the change in arterial pressure or a backward transmission from the central venous system remains unclear. (see Nilsson, Macrocirculation is not the sole determinant of respiratory induced variations in the reflection mode, Physiological Measurement [0967-3334] 2003; 24:935).